Dysnatremias (DN) are common electrolyte disturbances in cardiac critical illness and are known risk factors for adverse outcomes in certain populations. Little information exists on DN in children with cardiac disease admitted to the cardiac intensive care unit (CICU) after undergoing cardiac surgery, either corrective or palliative. The aim was to determine the incidence and adverse outcomes associated with DN in neonates and infants undergoing cardiac surgery. Retrospective cohort and single center study performed at Children’s Hospital Colorado from May 2013 to May 2014, in children under 1 year old admitted to the CICU after undergoing surgery for congenital or acquired cardiac disease. 183 subjects were analyzed. Exclusions: subjects that demonstrated DN before surgery. Serum sodium levels were recorded for the first 72 h post-operatively. DN was present in 54% of the subjects (98/183): hypernatremia in 60 (33%), hyponatremia in 38 (21%). Multivariate analysis revealed that mild hypernatremia (146–150 mmol/dl) and moderate hypernatremia (151–155 mmol/dl) were associated with longer hospital length of stay (LOS, p < 0.05) and ventilation times (p < 0.05). No association was shown between mild/moderate hyponatremia (125–134 mmol/dl) with either outcome. Hours to DN were significantly lower in hypernatremic (median = 5.8 h) than hyponatremic (median = 43.8 h) patients (p < 0.001). Children younger than 30 days presented DN at an earlier stage than those 31 days–1 year old (median +2.2 vs. 17.3 h). No associations present between DN and the class of diuretic (loop vs. thiazide) administered, or the route of administration (intravenous bolus vs. constant infusion). Total median sodium bicarbonate administration was associated with hypernatremia, as was exposure to vasopressin within the first 72 h post-operatively. Dysnatremias are common in the early post-operative period in neonates and infants undergoing cardiac surgery. Mild to moderate hypernatremia, but not hyponatremia, is associated with longer LOS and longer ventilation time in infants undergoing cardiovascular surgery. Hypernatremia is also associated with younger infants, a higher surgical complexity, administration of bicarbonate and exposure to vasopressin. Diuretic type or interval timing of intravenous delivery did not demonstrate any effect. Prospective studies are needed in this population, in order to determine how DN, particularly hypernatremia, contributes to adverse outcomes, whether this association is independent of illness severity, and what may be safe treatments and interventions for these disorders.